The influence of substituent groups on the resonance stabilization of benzene. An ab initio computational study

Accurate G3(MP2) calculations of the enthalpies of formation (Delta(f)H298) of organic molecules permit replication and extension of calculations that were formerly dependent on experimental thermochemical results. A case in point is Kistiakowski's classical calculation of the total stabilization enthalpy of benzene relative to that of cyclohexene, called for many years the "resonance energy". This paper investigates extension of the classical calculation to substituted benzenes. Slight modification of the usual procedure for Delta(f)H298 determination permits exclusion of all empirical information, leaving a purely ab initio result. Stabilization enthalpies relative to the corresponding 4-substituted cyclohexenes are presented for benzene, toluene, aniline, phenol, phenylacetylene, styrene, ethylbenzene, and phenylhydrazine. In the process of calculating these stabilization enthalpies, we have also obtained 42 values of Delta(f)H298 for monosubstituted benzenes, cyclohexenes, and cyclohexanes, 24 of which are not in the standard reference literature. For the remaining 18 G3(MP2) results, the unsigned mean difference between calculated Delta(f)H298 values and experimental results is +/-0.91 kcal x mol(-1).     

Geometrical structure of yttrium and metal-bromine complexes in solution: limitations of extended X-ray absorption fine structure analysis (EXAFS)

An extensive study on the appearance of multi-electron features in the X-ray absorption spectra of several yttrium(III)-based compounds has been performed. The existence of a multi-electron transition of non-negligible intensity within the extended X-ray absorption fine structure (EXAFS) region of the Y K-edge spectra has been proven. The impact of such features in the EXAFS analysis is made evident for aqueous solutions of YBr3 x 6H2O in liquid and glassy states in the concentration range 0.005-2.0 M, in which this transition induces an overestimation in the coordination numbers derived from EXAFS. We have performed theoretical computation of cross-sections for the double-electron processes at the K-edge of both Y and Br. These computations have been applied to the experimental EXAFS K-edge spectra of both Y and Br in several solids and in aqueous solutions. While in the case of Y K-edge spectra the presence of such multi-electron transitions was seen to seriously affect the standard EXAFS analysis, its influence in the case of Br K-edge spectra was determined to be negligible.     

Decarboxylierung cyclischer und acyclischer dicarboxylato-komplexe des platins

The interaction of (Ph₃P)₂PtO₂ (I) with the dicarboxylic acids HO₂C(CH₂)_nCO₂H (n = 1–3), phthalic acid and maleic acid gives the dicarboxylato complexes (Ph₃P)₂$\text{PtO(O)C(CH}{}_2\text{)}{}_{\mathrm{<mtext>n</mtext>}}\text{C(O)O}$ (II) (n = 1–3), (Ph₃P)₂$\text{PtO(O)CC}{}_6\text{H}{}_4\text{C(O)O}$ (III) and cis-[(Ph₃P)₂Pt(O(O)CCHCHC(O)OH)₂] (IV) in nearly quantitative yield. Thermal and photoinduced decarboxylation of III and IV yields the platina heterocycles (Ph₃P)₂$\text{PtC}{}_6\text{H}{}_4\text{C(O)O}$ (V) and (Ph₃P)₂$\text{PtCHCHC(O)O}$ (VI) with a carbon-platinum σ-bond. Complex VI has been characterized by an X-ray crystal structure determination.     

The determination of dissolved manganese and cadmium in sea water at low nmol 1−1 concentrations by chelation and extraction followed by electrothermal atomic absorption spectrometry

The development and application of a method suitable for the determination of dissolved manganese and cadmium in sea water at the low concentrations typical of the open ocean is described. A mixed dithiocarbamate/Freon TF extraction system is used to separate trace metals from major constituents, prior to back extraction into a final dilute nitric acid solution and subsequent determination of the metals by graphite-furnace atomic absorption spectrometry. The procedure also separates dissolved Co, Cu, Fe, Ni, Pb and Zn from a sea-water matrix. A mechanized system for the extraction step and measures to control contamination are described. The detection limits (3 σ) for manganese and cadmium are 0.10 and 0.04 nmol l^?1, respectively.     

Reactions of pyrrolidine enamines of cyclic and acyclic 3,4-dioxobutanoic acid Derivatives with dimethyl acetylenedicarboxylate. A new case of atropoisomerism.

Reaction conditions and structure of the starting enamines (cyclic or open-chain) determine greatly the final products of the title reactions. Whereas in benzene and acetonitrile, DMAD and 1 give a mixture of the diastereoisomeric dienamines 5, in methanol they afford pirrolizine 3. Enaminofuranones 2 and 10 furnish the corresponding “Michael adducts” 7a,b,c and 11a,b,c but fail to yield pirrolizines. It has been demonstrated that above b and c adducts differ exclusively on the arrangement of groups around a chiral axis.     

Dual-phase gas-permeation flow-injection thermometric analysis for the determination of carbon dioxide

A flow-injection configuration based on a dual-phase gas-permeation system from a liquid donor to a gas acceptor stream with a thermistor flow-through detector is proposed for the direct analysis of the gas in the acceptor. This system was applied for the determination of carbon dioxide (in the form of carbonate) using the following chemical reaction: CO(2)(g)+2NH(3)(g)+H(2)O(g)=(NH(4))(2)CO(3)(s), with a linear response from 1x10(-3) to 50x10(-3) mol l(-1) of CO(3)(2-). Carbon dioxide was produced in the liquid donor and permeated into the gaseous acceptor stream of air/water vapor. The detection limit is 1x10(-3) mol l(-1) of carbonate, and a sampling frequency of 60 h(-1) is achieved with a relative standard deviation of 4.1% for replicate injections. The dual-phase gas-permeation flow-injection manifold, along with the membrane and phase separations, as well as the chemical reaction, provides enhanced selectivity when compared with the system employing a liquid acceptor stream, as serious interferents in this system, for instance, acetate and formate, among others, do not interfere in the proposed system.     

Homolytic reactions of polyfluoroaromatic compounds V. The reaction of pentafluorophenyl radicals with fluorobenzene

The arylation of fluorobenzene with pentafluorophenyl radicals obtained by the reaction of the corresponding aniline with pentyl nitrite is described. The reaction involves attact at the 2t?-,3t?-and 4t?-positions. The composition of the mixture and the nature of its components were determined by ¹⁹F NMR spectroscopy and were confirmed by gas-liquid chromatography.     

Synthetic thermally reversible gel systems. V

Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity.     

Elastic properties of crosslinked poly(vinyl alcohol) gels: Network topology

Current network theory exhibits inconsistencies which show up particularly clearly in deformation of networks prepared by crosslinking a polymer in solution. A check of theory can be obtained if one knows precisely the number of crosslinks in the network and if a range of deformations is applied to the network. In an effort to explore this problem we have examined the relation of shear modulus to crosslink density, primary molecular weight, and polymer concentration for a series of poly(vinyl alcohol) gels at low to intermediate concentrations. Aqueous poly(vinyl alcohol) solutions were crosslinked to form infinite networks using terephthalaldehyde. We find a large discrepancy with these poly(vinyl alcohol) gels between measured shear modulus and that calculated from classical elasticity theory assuming quantitative reaction of crosslinking. The ratio of measured to calculated modulus is independent of crosslink density for a given primary molecular weight and concentration. It shows linear dependence on polymer concentration prior to crosslinking and extrapolates to a critical concentration which is consistent with the effective sizes of the polymer molecules.     

Influence of extended π-conjugation units on carrier mobilities in conducting polymers

Carrier mobilities in thin films of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of doping levels, where the numbers of thienylenes in the repeat unit are 7, 8, 10, 12, and 14. The mobilities for these polymer films increased with the increase in doping level and the mobility enhancement followed an increasing order of the π-conjugation length. The magnitude of the mobility increase for the Si polymer comprising 14 thiophene units reached ca. 10⁴, implying that this π-conjugation length is almost sufficient to reproduce transport properties of polythiophenes.